Hi Jakimo,
You may benefit from setting up Accounts rather than leads as it easier to sort out and search for.
You can certainly change a mandatory field. If you want to keep Last Name field you can just change 'Requirement Level' in Form customization to non-mandatory (this is how I did it anyway).
I am not sure about notes/activities. How/where do you add those? For example, I am not able to associate my activities with leads(when looking at list of activities) but you can assign it to an Account or a Contact. This is why it may be better to set up Accounts first.
Please ask if you have any questions
Monika